Gas hydrates have been known for many years. These hydrates are inclusion compounds wherein various light hydrocarbon gases or other gases, such as natural gas, associated natural gas, methane, ethane, propane, butane, carbon dioxide, hydrogen sulfide, nitrogen, and combinations thereof, physically react with water at elevated pressures and low temperatures. The gas becomes included or entrapped within the extended solid water lattice network which includes hydrogen bonded water molecules. The hydrate structure is stable due to weak van der Waals' forces between the gas and water molecules and hydrogen bonding between water molecules within the lattice structure.
An exemplary, non-stoichiometric reaction equation for the formation of natural gas hydrates is as follows: EQU CH.sub.4 (g)+mH.sub.2 O(l).fwdarw.CH.sub.4 (H.sub.2 O).sub.m (s).
In this equation, the value "m" typically is 4 to 6, and the heat of formation (.increment.H.sub.f) is -410 kJ/kg hydrate for methane hydrate, which is approximately 25% higher than the heat of fusion of water. The reverse reaction, exploited during regassification, is endothermic. Because gas hydrates are solids that form at a gas-water interface, the formation and regassification reactions are mass-transfer limited.
At least two different hydrate crystalline structures are known, each of which is a clathrate crystalline structure. A clathrate hydrate unit crystal of structure I includes two tetrakaidecahedron cavities and six dodecahedron cavities for every 46 water molecules. A clathrate hydrate unit crystal of structure II contains eight large hexakaidecahedron cavities and 16 dodecahedron cavities for every 136 water molecules. A relatively large volume of gas can be entrapped under pressure in these cavities. For example, it has been determined that natural gas hydrates can contain as much as 180 standard cubic feet of gas per cubic foot of the solid natural gas hydrates.
Early on, gas hydrates were considered an industrial nuisance. Petroleum and natural gas production facilities often are located in cold environments, where the product is located in deep underground or underwater wells. When tapping these wells, all of the necessary conditions and ingredients are present for producing gas hydrates--i.e., light hydrocarbon gases and water are present, the temperature is low, and the pressure is high. Therefore, gas hydrates often would be produced spontaneously in the drilling and transmission pipes and equipment when an oil or natural gas well was tapped. Because gas hydrates are solid materials that do not readily flow in concentrated slurries or in solid form, when they are spontaneously produced in oil or natural gas production, they tend to clog the equipment, pipes, and channels in the production and transmission systems. These disadvantageous properties of gas hydrates spawned much research into methods for inhibiting hydrate formation and eliminating this nuisance. See, for example, D. Katz, et al., Handbook of Natural Gas, McGraw-Hill, New York (1959) pp. 189-221; E. D. Sloan, Jr., Clathrate Hydrates of Natural Gases, Marcel Dekker, Inc. (1991). These documents are entirely incorporated herein by reference.
But, because of the relatively high volume of gas that potentially can be stored in gas hydrates, eventually researchers began to look at this "nuisance" as a possible method for safely and cost effectively storing and/or transporting gases. See B. Miller, et al., Am. Gas. Assoc. Mon. Vol. 28, No. 2 (1946), pg. 63. This document is entirely incorporated herein by reference. Several researchers and patentees have described methods of producing gas hydrates. See, for example, U.S. Pat. No. 3,514,274 to Cahn, et al., which document is entirely incorporated herein by reference.
While there is extensive documentation relating to gas hydrate production processes, less attention is paid in the literature to devices and methods for storing and regassifying the hydrates. These aspects of gas hydrate production also are important. If the gas hydrates cannot be conveniently, reliably, and inexpensively stored for extended time periods, the production thereof is of limited usefulness. Additionally, if the gas hydrates cannot be conveniently and controllably regassified, there is little or no point to producing and storing the hydrates.
Hutchinson, et al., U.S. Pat. No. 2,375,559 (which patent is entirely incorporated herein by reference), describe a process for hydrating hydrocarbon gases and storing the produced hydrates in storage tanks. Few details are provided in Hutchinson relating to the manner in which these stored hydrates are regassified.
U.S. Pat. No. 2,904,511 to Donath illustrates a water desalination apparatus that produces desalinated water from salt water by forming gas hydrates. Because this patent relates primarily to a desalination method, hydrate storage and gas recovery is not a concern of Donath. Rather, the hydrates are passed immediately into a hydrate decomposition vessel where the gas is liberated from the relatively desalinated water present in the hydrate. This Donath patent also is entirely incorporated herein by reference.
Gudmundsson also describes various systems for producing gas hydrates. See, for example, U.S. Pat. No. 5,536,893; WO Patent Publication No. 93/01153; "Transport of Natural Gas as Frozen Hydrate," ISOPE Conference Proceedings, VI, The Hague, Netherlands, June 1995; and "Storing Natural Gas as Frozen Hydrate," SPE Production & Facilities, February 1994. These documents each are entirely incorporated herein by reference. U.S. Pat. No. 5,536,893 describes agglomerating the gas hydrates into solid blocks suitable for long term storage at atmospheric pressure and at a temperature below 0 to -15.degree. C. Few details are provided concerning the method and apparatus used for hydrate storage and regassification.
Gudmundsson discloses storage of gas hydrates under "metastable" conditions, i.e., conditions under which one would normally expect the hydrates to be unstable and decompose. Under these relatively mild metastable conditions (5 to 20.degree. F. and ambient pressure), natural gas hydrates dissociate sufficiently slowly to remain intact for periods of time suitable to ocean transport or large-scale storage (e.g., for 10 days or more). This metastability phenomenon is attributed to spontaneous regassification of the outer surface of a macroscopic hydrate sample. Because the hydrate regassification process is endothermic, once the outer surface of the hydrate sample dissociates, auto-refrigeration freezes the dissociated water to create an ice shell that significantly insulates the bulk hydrates and attenuates the mass transfer rate of gas from within the interior of the sample.
Additionally, gas hydrates are effective insulators (thermal conductivity "k" of about 0.5 W/m K for hydrates, as compared to a thermal conductivity "k" of 2 for ice). This insulative property helps keep bulk gas hydrates from dissociating too rapidly. The metastability and the insulative properties of gas hydrates allow them to remain stable under relatively mild conditions after they are initially produced.
With regard to hydrate dissociation methods, Rogers, et al., "Hydrates for Storage of Natural Gas," Mississippi State University, Proceedings of the Second International Conference on Natural Gas Hydrates, June 2-6, 1996, Toulouse, France, describe an ultrasonic dissociation concept that was intended for use in vehicular applications.
Traditionally, hydrate-forming gases, such as natural gas, associated natural gas, methane, ethane, propane, butane, carbon dioxide, nitrogen, and hydrogen sulfide, have been stored under high pressures. Liquefied-natural gas and liquefied propane are examples of this type of storage system. Because of the presence of high pressure cylinders, storage of gases under high pressures and liquefied conditions presents a significant safety issue and is very expensive.